Fix tick spacing calculation

[kurkle]

Fixes: #6759

@benmccann do you recall why that - 1 is there?

[benmccann]

Yeah, here's an old comment that maybe should have been left:

Chart.js/src/core/core.scale.js

Line 856 in daafbc7

// Total space needed to display all ticks. First and last ticks are

Basically we're counting the spaces between ticks and figuring out how to divide that

[kurkle]

This is actually the index spacing to ticks array to make enough fit. So not quite same thing anymore.

[benmccann]

I see we have a + 1 already that probably accounts for this with the ticks at the end hanging off the edge:

Chart.js/src/core/core.scale.js

Line 916 in eb0751c

var ticksLimit = tickOpts.maxTicksLimit || axisLength / me._tickSize() + 1;

So yeah, it might be the case that we're doing a +/- 1 adjustment an extra time